The automotive industry has made remarkable contributions to world economy and human mobility, and automobiles have been viewed as a boon to individualism, freedom and liberation of modern society. The use of automobiles consumes considerable amounts of fossil fuels and it becomes an important source of environmental impact. About 70% of oil consumed serves for transportation and more than half of the contaminants come from tail gases of vehicles. The development of electric vehicles (EVs) is considered as one of the solutions to those problems and many different EVs have been introduced to marketplace in recent years and have become increasingly popular.
It is well known that EVs have a lot of advantages over the conventional motorcar, such as an easy operation, no emissions of foul odors or gases and a quiet ride.
As road vehicles, the state-of-the-art EVs have three major problems: a limited travel range, a slow start speed and a high price. Those problems are determined by the energy storage systems (ESSs) of the vehicles. The most widely used ESSs of EVs include batteries, fuel cells and electrochemical capacitors (ECs). A battery is comprised of one or more electrically connected electrochemical cells having terminals/contacts to supply electrical energy, and an EC is a device that stores electrical energy in the electrical double layer which forms at the interface between an electrolytic solution and an electronic conductor.
Each ESS has its own advantages and drawbacks. EC and Li-ion battery (LiB) are introduced as ESSs for EVs because they can provide a high energy density or a high power density. These devices are at two ends of the energy storage spectrum. In a LiB, the lithium ions move from the negative electrode to the positive electrode during discharge, and this process can provide a high chemical energy. However, the power density is quite low due to a low speed of Li-ions movement and intercalation. Although the LiB has a high energy density from 120 to 200 Wh/kg, the power density is only about 100 W/kg. But in an EC, the main source of energy results from the electrolyte adsorption/desorption on the electrodes. This process is extremely fast in comparison with the chemical reaction process of a LiB. Therefore, ECs have a high power density, ranging from 2 to 10 kW/kg or more, because of the relatively high speed of Li-ions absorption/desorption, but the energy density is only 2 to 5 Wh/kg. Therefore, it was proven difficult for a traditional ESS to achieve the demands of both energy density as well as power density for EVs.
To solve those problems, researchers have proposed different solutions. One of the most widely used approaches is to develop a hybrid system consisting of an EC electrode and a battery electrode [Lipka S. M., Reisner D. E., Dai J., Cepulis R., in Proceedings of 11th International Seminar on Double Layer Capacitors, Florida Educational Seminars, Inc., Boca Raton (2001). Amatucci G. G., Badway F., Pasquier A. D., Zheng T., J. Electrochem. Soc. 0013-4651, 148, A930 (2001). Zheng J. P., Jow T. R., J. Electrochem. Soc. 0013-4651, 142, L6 (1995)]. In this structure, the positive electrode stores charge through a reversible non-faradaic reaction of anions and the negative electrode utilizes a reversible faradic reaction of lithium-ion insertion/extraction in a nano-sized lithium-ion intercalated compound. Compared with traditional ECs, the hybrid EC shows a higher energy density. Telcordia Technologies has proposed a new device named nonaqueous asymmetric hybrid electrochemical supercapacitor (HBEC) with an intercalation compound Li4Ti5O12 as the negative material and active carbon as the positive material. Glenn G. Amatucci Fadwa Badway, Aurelien Du Pasquier, Tao Zheng, “An Asymmetric Hybrid Nonaqueous Energy Storage Cell”. Journal of The Electrochemical Society, 148(8)A930-A939 (2001). However, the energy density of these devices is too low to be used as ESS for EVs, and more work still remains to be done on this topic.
It has been previously reported that a novel structured lithium-ion capacitors (LiCs) can be provided by replacing the conventional activated carbon anode with a hard carbon (HC) anode covered by a stabilized lithium metal powder (SLMP) layer on surface [W. J. Cao, Y. X. Li, B. Fitch, J. Shih, T. Doung, J. P. Zheng, “Strategies to optimize lithium-ion supercapacitors achieving high performance: Cathode configurations, lithium loadings on anode, and types of separator”, Journal of Power Sources, 268, 841 (2014). W. J. Cao and J. P. Zheng, “Li-ion Capacitors with Carbon Cathode and Hard Carbon/SLMP Anode Electrodes”, J. Power Sources, 213, 180 (2012). 2. W. J. Cao, J. S. Zheng, D. Adams, J. P. Zheng, “Comparative Study of the Power and Cycling Performance for Advanced Lithium-Ion Capacitors with Various Carbon Anodes”, J. Electrochem. Soc., 161, A2087 (2014). The added SLMP layer can increase the open circuit voltage of the EC and ensure less salt to be consumed when it is charged. This LiC is capable of storing approximately 5 times more energy than conventional ECs and has the benefit of a high power density. Although the LiC can improve the energy density, it is very difficult to improve the energy density of a LiC too much because of chemical properties, and as a consequence achieving the high energy density requirement of EVs is still difficult to attain.